1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus, and more particularly, to an optical scanning device that scans a scanning surface by a light flux and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
Recently, while prices of image forming apparatuses such as an optical printer, a digital copier, and an optical plotter have become less expensive, a high stability for temperature change has been required for these apparatuses. These image forming apparatuses include an optical scanning device that scans a scanning surface by a light flux from a light source.
With the advent of a high precision machining technique, as a method of achieving a high stability, a low price, and a reduced number of components, the use of an optical element (a diffractive lens, a phase shifter, a sub-wavelength structure (SWS) element, and the like) having a fine shape is considered.
When the diffractive lens is used for the optical scanning device, an advanced function and multiple functions can be achieved with a small number of components, and thus, the diffractive lens is expected to achieve a high precision of an optical characteristic as well as a great effect for the reduction in size of the optical scanning device.
For example, Japanese Patent Application Laid-open No. 2005-258392 discloses an optical scanning device including: a light source formed of a semiconductor laser; a coupling optical system that couples a light flux from the light source; a first optical system that converts the light flux from the coupling optical system in a main scanning direction into a parallel light and that converges the light flux in a sub-scanning direction onto a deflecting unit; a deflecting unit that deflects the light flux from the first optical system in the main scanning direction; and a scanning optical system that concentrates the light flux deflected by the deflecting unit, in which, materials of all lenses configuring the coupling optical system are resin, and at least one surface of the lenses is formed with a diffractive optical surface.
Japanese Patent Application Laid-open No. 2002-287062 discloses a laser scanning apparatus including: a laser light source that emits a laser beam; a deflecting unit that deflects an incoming laser beam into a main scanning direction; a light-source optical system that converts the laser beam, in the main scanning direction, emitted from the laser light source into substantially a parallel light and that concentrates the laser beam in a sub-scanning direction near a deflecting surface of the deflecting unit; and a scanning optical system that concentrates again the laser beam deflected by the deflecting unit, in which the light-source optical system is formed of one optical element configured by resin, and the optical element includes at least one surface of a reflecting surface having no axis of rotation symmetry, and two surfaces of transmitting surfaces (of which the two surfaces are both diffractive surfaces, and on the two diffractive surfaces, changes in diffractive angle at the time of a wavelength change are configured to be opposite to each other).
Japanese Patent Application Laid-open No. 2004-126192 discloses an optical scanning device including: a light source unit; an optical unit that guides a light flux from the light source unit to an optical deflecting unit; an imaging optical system that guides the light flux from the optical deflecting unit to a scanning surface; and the scanning surface being optically scanned based on a rotary operation of the optical deflecting unit, in which the optical unit has a diffracting unit on one or more surfaces, and a specific expression including a focal length, a spot diameter, an oscillation wavelength, a power, and a dispersion value satisfies a specific condition.
A diffractive lens having a trace step corresponding to a phase difference of equal to or more than 2π can impart, similar to a refractive lens, functions of refracting a light flux, concentrating light, and so on. A property of the diffractive lens, which differs from that of the refractive lens, includes a strong negative dispersion. When the property of the diffractive lens and a wavelength change of a light source concurrently with a temperature change of an optical system are appropriately combined, a so-called temperature compensating function can be realized.
The temperature compensating function can be realized when a change in optical characteristic resulting from the temperature change of the optical system and the wavelength change of the light source are generated in good balance. Accordingly, when a laser light source, which is represented by a semiconductor laser diode (LD), is used, a deterioration of geometric aberration resulting from a wavelength variance of the light source, such as a wavelength difference depending on each light source element, a wavelength transition (mode hopping) during emission, a wavelength difference between light-emitting units in an array element need be taken into consideration. This is considered to be an inevitable issue caused as a result of bringing an optical wave characteristic into a geometric aberration correction.
As described above, when the temperature change is generated, the diffractive lens having the temperature compensating function realizes its function by balancing: (1) the negative dispersion characteristic of the diffractive lens caused by the wavelength variation of the light source; and (2) a focus deviation caused by a thermal expansion of the optical elements. Particularly, a dominant factor in (2) is the thermal expansion of a scanning lens included in the scanning optical system. It can therefore be said that designing the temperature compensating function of the diffractive lens is achieved by balancing the negative dispersion characteristic and an amount of the temperature change of the scanning optical system.
In an actual optical scanning device, the temperature change is not generated evenly within the apparatus. The reason for this is that heat sources are independently operated, i.e.: (A) heating resulting from driving the light source; (B) heating resulting from driving the deflecting unit; and (c) heating resulting from a heat source outside of the optical scanning device.
Particularly, the heating in (B) is generated for driving the deflecting unit at high speed, and is the most dominant heating of all the temperatures within the optical scanning device. Generally, the heating in (A) is generated by driving an electric circuit, and thus, the amount of heating is much smaller as compared to that in (B).
Thus, resulting from the independent heat sources and the difference in amount among these heat sources, an uneven temperature distribution is generated within the apparatus while the optical scanning device is driven. In practice, the temperature distribution is such that at the center of a deflecting unit having the largest amount of heating, the heat is diffused within the optical scanning device. Such a temperature distribution is a complicated phenomenon relating to a shape of a casing and an air current, and thus, estimating in advance at the time of optical design is very difficult. This results in a current situation such that when designing the diffractive lens having the temperature compensating function, a specific pattern is merely estimated, e.g., to rely on “when the temperature is changed evenly within the optical scanning device”.
The generation of the uneven temperature distribution means a generation of a temperature difference, i.e., the temperature is higher near the deflecting unit, and the temperature is lower in the optical element apart from the deflecting unit. The diffractive lens having the temperature compensating function is so designed to estimate that the wavelength change of the light source and the change resulting from the temperature of the scanning optical system are balanced, and thus, the generation of the uneven temperature distribution breaks down the balance.